The U.S. government shutdown has taken some government Web sites offline, including data.gov. But the nation's most powerful supercomputers continue to operate -- for now, at least.

Government supercomputers are running on reserve funds from last year, a U.S. Department of Energy spokesperson said. How long can these systems continue to do so? "That fact is unknown at this point in time," the spokesman said.

The Energy Department's national laboratories run petascale supercomputers used in scientific research.

The national labs are technically contractors and have some budget flexibility that other government agencies do not. But one source said that there is contingency planning to conserve funding, including canceling some travel and an extreme-scale research conference that was due to start next week.

A national lab source said officials are deciding what do if the shutdown goes on much longer. At some point, "it may come down to choosing what equipment to keep powered up, and if employees should be furloughed."

Researchers across a wide range of disciplines are granted time on government systems to investigate scientific problems. They have to compete for time on these systems, and, if approved, are granted a certain number of core-hours to run simulations.

The systems available for scientific research include Titan -- the world's second most powerful system as ranked by the Top 500 list. It is a Cray system capable of 17.59 petaflops and operates at the Energy Department's Oak Ridge National Laboratory. There are systems almost as powerful at Lawrence Livermore Laboratory and Argonne National Laboratory.

China's Tianhe-2, at 33.86 petaflops, is currently the most powerful, according to the Top 500.

The response of government websites to the shutdown that began at 12:01 a.m. Tuesday has been inconsistent. While data.gov is offline, the government mobile apps store appears operational. The U.S. Department of Agriculture, for instance, has pulled its website, while the Department of Energy has not.